The typical fluid treatment element may comprise a hollow, generally cylindrical, permeable cartridge and an end cap positioned on each end of the cartridge to direct a fluid, i.e., a gas or a liquid, through the cartridge. Depending on the specific treatment to be given the fluid, the cartridge may, for example, include a filter medium for removing particulates or a demineralizer or sorbent for separating ionic or chemical components.
Frequently, the fluid is forced to flow outwardly from the interior to the exterior of the hollow, permeable cartridge. This "inside-out" flow may be the normal direction of flow through the element, because it may have certain advantages. For example, many filters have a graded porosity with the coarsest pores on the upstream portion of the filter and the finest pores on the downstream portion of the filter. If the normal direction of flow is inside-out, the downstream portion of the cartridge where the pores are finest is the exterior portion of the cartridge where the diameter of the cartridge is greatest. This helps to reduce the pressure drop across the finest porosity portion of the filter
However, even if the normal direction of flow is "outside-in", the direction of flow may occasionally be reversed. This reversal of the normal outside-in flow may be accidental, e.g., due to a surge in fluid pressure downstream from the element, or it may be intentional, e.g., to flush an accumulated cake of particulate matter from the external surface of the cartridge.
Whenever the fluid flows inside-out through the permeable cartridge, the fluid exerts an outwardly-directed force on the interior of the cartridge. If this force is large enough, the cartridge may distend, distort, rupture, or even burst, thereby reducing the efficiency of the cartridge or even rendering the cartridge totally ineffective. Many high efficiency filter cartridges characterized by low resistance to inside-out flow include a medium of fibers that are not tightly bonded to one another. The force required to distort such a cartridge may be relatively small.